Audio Amplifiers
The present invention relates to a pulse width modulation amplifier circuit, and more particularly, to a differential input Class D circuit in which good circuit performance in terms of Power Supply Rejection Ratio (PSRR), Noise, and Total Harmonic Distortion (THD) is achieved.
Most of audio power amplifiers in the market are based on Class AB amplifier. Class AB offers very good total harmonic distortion plus noise (THD+N) performance, with fairly low quiescent current. However, the Class AB push-pull amplifiers are very inefficient and can only achieve an efficiency of about 60%, which results in not only power loss, but also additional bulky heatsink attached to the power amplifiers.
With the advance of fabrication techniques, making integrated Class D audio power amplifier becomes possible. One major advantage of Class D amplifiers is the efficiency, which could reach above 90%. The high efficiency is achieved by full signal swing at power transistors. In a conventional simple Class D amplifier system, the analog input signal such as music signal is converted to a pulse signal, and then this pulse signal is split and passes through level shifter and driver stage to drive output power transistors. The output terminal of the amplifier is connected to the input terminal of the load, such as a loudspeaker via a low-pass filter. Many Class D amplifiers use pulse width modulator to generate pulse trains which vary pulse width in proportion to the audio signal's amplitude. However, some Class D amplifiers may also be configured with other types of pulse modulators, for example, pulse density modulator and self-oscillating modulator.
A balanced transformer-less (BTL) Class D amplifier with feedback circuit 100 is given in FIG. 1, which includes a preamplifier 12, a summing amplifier 14, a triangular wave generator 24, a filter 20, a comparator 22, a latch 28 and an output stage 16. After the input analog signal passing through preamplifier 12, it is applied to the positive port of summing amplifier 14. Output signal is fed back to the negative port of summing amplifier 14. The generated signal from summing amplifier 14 passes through a filter 20, and then is applied to the positive input of comparator 22. The negative input to the comparator 22 is generated by a triangular wave generator 24. The output of the comparator 22 is therefore high when the input signal is higher than the value of the triangular wave 25, and low when the input signal is lower than the value of the triangle wave 25. The output of the comparator 22 is a pulse train with a duty cycle proportional to the instantaneous input signal level. This pulse train is input to a latch 28 which converts the single ended comparator output to a differential signal input to output stage 16, which in turn drives loudspeaker 18. The latch 28 is to ensure no high frequency oscillations, which may occur in the frequency range of comparator state transitions. The latch 28 also ensures that two pulses driving output stage 16 never overlap.
One disadvantage of above BTL Class D amplifier with feedback circuit 100 is its input port configuration, which is only compatible with single-ended audio source. It can not be used for differential input audio source.
To overcome this disadvantage, one method is to design a Class D amplifier with differential input. A simple new architecture with good system stability has to be introduced to provide good noise, Total Harmonic Distortion (THD) and Power Supply Ripple Rejection (PSRR) performance.
Fully Differential Error Amplifier
A difference amplifier is shown in FIG. 2, which is well known in many publications. The differential gain is obtained by proper setting of R1 701, R2 702, R3 703, R4 704 values. However, this commonly used difference amplifier cannot be used for differential output application. Therefore, a fully differential error amplifier has to be designed to cater for the differential input signal, differential output signal and differential feedback signal with proper dc bias design.
Pop Noise Suppression
In an audio amplifier circuit, including Class D amplifier circuit, when turning on the power supply, an unpleasant abnormal sound called pop noise is generally produced and in an even worse scenario the overcurrent protection circuit may be triggered by this generated pop noise.
The pop noise cause for differential Class D amplifier architecture varies. In the proposed differential input Class D amplifier in FIG. 7a, with the absence of the anti pop noise technique, first bias voltage 603 rises up from 0V when startup. As the bias voltage for different blocks are not high enough to enable Class D amplifier to operate normally. The differential output from fully differential error amplifier 280, first error signal 281 and second error signal 282, are not stably defined. They are switching between high and low randomly. Therefore, buzz noise or pop noise at the loudspeaker is heard.
For the same reason, in case of shutdown buzz noise or pop noise at the loudspeaker is heard when bias voltage for different blocks drop to a level that Class D amplifier is not able to operate normally.
Therefore, a method to remove pop noise during startup and shutdown has to be proposed.